Display devices



Oct. 23, 1962 E. A. sAcK, JR 3,060,345

DISPLAY DEVICES Filed oct. 25, 1960 WWW] M |NvENToR Edgar A. Sack,Jr.

ATTORNE WITNESSESI vania Filed Oct. 25, 1960, Ser. No. 64,944 11 Claims. (Cl. 315-169) This invention relates to display devices for the presentation of a light image which may be amplified in comparison with an input radiation image, and particularly to such devices which utilize electroluminescent cells for the source of output light.

Devices are Well known in the art, such as that described in U.S. Patent 2,650,310, issued August 25, 1953, which comprise a stratum of photoconductive material and a stratum of electroluminescent phosphor material sandwiched together between two electrodes across which an A.C. potential is applied. Under dark conditions, when no energy to be amplified impinges upon the photoconductive material, the impedance of the photoconductive layer is high and the voltage drop thereaeross, being directly related to impedance, is also high. Therefore, the device parameters may be selected so that under dark conditions there is not sufcient voltage across the electroluminescent phosphor material to cause it to emit any noticeable amount of light. When energy, such as X-rays or visible light, impinges upon the photoconductive material, its impedance decreases in relation with the impinging energy and more of the applied A.C. voltage appears across the electroluminescent phosphor material causing it to emit light because of the increased A.C. electric field. The photoconductive material is sensitive to impinging radiation on localized areas, that is, if an image is imposed on a layer of photoconductive material the impedance of the material will decrease in accordance with the amount of energy striking each portion and the resulting field across the electroluminescent phosphor material will vary in a similar manner with the effect that a light image is obtained from the electroluminescent phosphor material.

ln order to achieve the device operating as aforesaid it is required that the impedance of the photoconductor be suiiciently high, under dark conditions, to keep the voltage drop across the electroluminescent portion of the device below the level at which substantial electroluminescence occurs. This requires that the thickness of the photoconductor be great compared to the stratum of electroluminescent phosphor material. However, a thick photoconductive layer results in impared resolution, insensit-ivity and in slowness of response.

It is therefore a general object of the present invention to provide an energy or image amplifying or display device having high resolution and high sensitivity and a short response time.

Another object is to provide an image amplifying device using light sensitive elements which have a high speed of response and a high dark impedance.

Another object is to provide an image amplifying device in which a thin sheet of photoconductive material may be used while still providing proper impedance conditions under a wide range of light levels.

According to the present invention, the electric field across an electroluminescent phosphor layer is varied in accordance with an applied radiation signal by means which includes for each display element at least one semiconductive control element which has a negative resistance and hyperconductive current-voltage characteristic. According to another feature of the present invention, such control elements are used as the light sensitive elements of a light amplifier or energy converter and in other embodiments such elements cooperate with a member of photoconductive material to alter the field across the electroluminescent layer.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with the above-mentioned and further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, in which:

FIGURE 1 is a partial perspective view, partly in section, of a light amplifying device in accordance with one embodiment of the present invention;

FIG. 2 is an enlarged partial sectional view of the device of FIG. l;

FIG. 3 is a curve of current v. voltage for the semiconductive control elements used in devices in accordance with the present invention;

FIG. 4 is a circuit schematic of a single element of the device of FIGS. 1 and 2;

FIG. 5 is a partial perspective view of a light amplifying device in accordance with another embodiment of the present invention;

FIG. 6 is an enlarged sectional view of the device in FIG. 5; and

FIG. 7 is a circuit schematic of a single element of the device of FIGS. 5 and 6.

Referring now to FIGS. 1 and 2 there is shown a light transmissive support member 10 of a material such as glass having thereon a front electrode 12 and a layer of electroluminescent phosphor material 14. The front electrode 12 should be of a light transmissive material such as tin oxide formed by techniques well known in the art. The electroluminescent layer 14 may comprise any of the known phosphor materials which exhibit electroluminescence upon being subjected to a time varying electric iield such as, for example, zinc sulfide activated by copper and chlorine. The electroluminescent layer 14 may be formed of granulated phosphor material embedded in a dielectric medium of a plastic or glass nature such as cyanoethylcellulose or, alternatively, it may be formed as a thin iilm of phosphor material deposited from a vapor without the usual dielectric medium.

On the back surface of the electroluminescent layer 14 there is disposed a plurality of elemental back electrodes 16 which may be arranged in a pattern of uniform rows as shown or in any other suitable pattern. The back electrodes 16 may be formed by evaporating a material such as aluminum or palladium through a suitable mesh or tin oxide electrodes may be formed as for the front electrode 12 if the electroluminescent phosphor is embedded in a material able to withstand the high temperature necessary.

The electroluminescent screen 11 thus far described consists of a plurality of individual elements defined by the elemental back electrodes 16 which make it possible to individually eXcite and control the electroluminescence from the localized phosphor areas adjacent the elemental back electrodes 16. On the back of the electroluminescent screen 11 is disposed a light sensitive control structure 18 which cooperates with the electroluminescent screen 1v1 to provide a light output related to the radiation input on the various elements of the screen.

The control structure comprises a carrier member 19 of a suitable rigid insulating material such as a ceramic known as steatite which is provided for convenience in fabrication and on which there is provided for each light producing element of electroluminescent cell 11 a pair of light sensitive semiconductive control elements 20 and 21 having a negative resistance and hyperconductive current-voltage characteristic as will be discussed more fully hereinafter. The support member 19, when made of opaque material, also prevents feedback of light from the electroluminescent layer 14 to the control elements 20 and 21. The support member has, in prole, saw teeth like projections on its upper surface. Two rows of such projections 22 and 23 extend across a single row of electroluminescent light producing elements. For a single element, one semiconductive control element is provided on each of the sloping surfaces of the support member and the control elements 20 and 21 are interconnected by a conductor 25 extending from the base of one element 21 to the emitter of the other 2G.

The structure is completed by conductive members 27 extending through the support member 19 connecting the base of one of the control elements 20 to the back electrode 16 of the electroluminescent cell 11. All the emitters of one of the pair of control elements 21 for each light element may be interconnected by conductors 29 and connected to one side of an A.C. voltage source 31 by lead 32. The other side of the A.C. source 31 is connected -to the front electrode 12 by conductive lead 33.

The semiconductor control elements 20 and 21 may each be of the type which is described in a copending application, Serial No. 649,038, filed March 28, 1957, by John Phillips and assigned to the same assignee as the present invention. The device therein described has three adjoining regions of germanium of alternate conductivity type, either pnp or npn, with a mass of metal attached to one of the outer semiconductive zones forming a device designated either as pnpm or npnm. A suitable device having7 Similar characteristics may be made of silicon and consist of four regions of alternating semiconductivity, either pnpn or npnp. As shown in FIG. 2 the two semiconductive control elements 20 and 21 in a single screen element are attached by a conductor 25 joining the bottom region of one element 21 with the uppermost semiconductive region of the other element 20.

Other semiconductor control devices are suitable for use as the control elements 20 and 21 and some are described generally in an article appearing in Electronics, February 27, 1959, by T. P. Sylvan. It is necessary that the device used have a characteristic of blocking the flow of current in one direction up to a certain voltage at which breakover occurs and a transition region of negative resistance is passed through and a subsequent stable region of hyperconductivity in which current increases with very little potential drop. It has been found that the potential at which breakover occurs is controllable by radiation incident on the device, particularly on the material near the uppermost pn junction. The incidence of radiation on or near this junction causes a substantial reduction in the potential necessary for breakover.

Referring now to FIG. 3 there is shown a current versus voltage characteristic curve for a pair of semiconductive control elements of the type referred to connected in back-to-back relationship, that is, so that the current providing switching must travel through the two elements in opposite directions. The portion of the curve in the rst quadrant represents the characteristic of a single element with voltage and current applied thereto in the forward direction, i.e., in the direction which produces the switching characteristic. The curve in FIG. 3 shows that each element has a high resistance region 41 and 41', a negative resistance region 42 and 42 in which the voltage drops very rapidly while the current remains substantially the same or is increasing only slightly, and a hyperconductive region 43 and 43 in which the current may increase substantially indefinitely and the element behaves simply as a closed switch. As shown in FIG. 3, if an alternating potential were applied across the two elements they would be conductive only during that portion of a single cycle in which the instantaneous potential exceeded the breakover potential (VBO) of the elements. Since radiation on the elements 20 and 21 reduces the breakover potential, such radiation will cause the elements to be conductive over a longer portion of the duty cycle.

Now with reference to FIG. 4 the operation of the device of FIGS. l and 2 will be discussed. Under dark conditions when there is substantially no radiation incident to the semiconductive control elements 20 and 21 there is no light output from the electroluminescent cell 11 because the A.C. supply 31 and the switching elements are selected so that the supply has a peakto-peak voltage of less than twice the breakover potential of one of the switching elements 20 and 21 in the dark, it being assumed that each element of the pair has substantially the same characteristics. Upon the application of radiation such as X-rays or visible light to the switching elements, the breakover potential is reduced to a value less than half of the peak-to-peak supply voltage and consequently light is produced from the electroluminescent cell 11 in accordance with the input radiation. For example, a source 31 having peak-topeak output of about volts may be used with elements 20 and 21 which have a dark breakover potential of about 8O volts each which can reduce to about 6 volts under light conditions.

The manner in which the light from the electroluminescent cell 11 varies in response to radiation on the control elements 20 and 21 depends on the relative values of the control element impedances and the capacitance of the electroluminescent cell and the frequency of the applied alternating potential. That is, the various circuit impedances determine the period of time (R-C time constant) required for the voltage to build up across lthe electroluminescent cell 11 after the control elements 20 land 21 become hyperccnductive. The relation between this time constant and the period of the applied alternating excitation has an important effect on device operation.

If it is the case that the R-C time constant is very short compared to the excitation period, then the voltage drop across the electroluminescent cell 11 during a single A.C. cycle essentially reaches the peak-to-peak excitation potential. During each cycle the maximum light output from the electroluminescent cell 11 is fixed regardless of the magnitude of input radiation, so long, of course, as the control elements do in fact breakover. Therefore, an essentially bistable device results in which the electroluminescent cell is on or off but is not capable of presenting half tones. While such a device has certain usefulness, the ability to display half tones is naturally a desired feature.

It is within the scope of the present invention to provide an improved display device of the type in question (FIGS. 1, 2 and 4) which does provide half tone display. If the R-C time constant is of the order of the excitation period, the voltage drop `across the electroluminescent cell 11 does not necessarily reach the peak-topeak excitation potential but reaches a potential determined by the radiation incident on the control elements 20 and 21. Upon breakover of the control elements, the voltage begins to build up on the electroluminescent cell. The voltage will continue to build up until the instantaneous excitation potential is reached but this is likely to occur after the A.C. potential has peaked since the time constant is relatively long. Since lthe instant of breakover is determined by the input radiation, the light output from the electroluminescent cell varies in accordance with the input radiation and half tone display results. Therefore, in the preferred form of the present invention the circuit parameters are chosen so that the R-C time constant of the circuit is of the order of the period of lthe applied excitation potential.

In the embodiment of FIGS. l and 2, the pair of control elements behaves similarly to a photoconductive element but may have a higher speed of response compared with conventional photoconductors, such as a layer of cadmium sulfide, by a few orders of magnitude. Another advantageous feature is that resolution is only limited by the necessary size of the structure to permit fabrication, that is, resolution is essentially limited only by the necessary size of the control elements and 21.

In Patent No. 2,916,630 issued December 8, 1959 to B. Rosenberg another means for avoiding the drawbacks of the use of a thick photoconductive layer -is described wherein the stratum of photoconductive material and the stratum of electroluminescent phosphor material are separated by an additional electrode. A D.C. potential is applied across the two strata by means of the outermost electrodes and commutation means is provided to rapidly switch the applied potential Ifrom being across both strata to being across only the photoconductive stratum. The capacitor comprising the electroluminescent layer and the electrodes adjacent thereto is charged up during the portion of the operating cycle in which the applied potential is across both the photoconductive layer and the electroluminescent layer. The extent of charging, i.e., the maximum voltage achieved across the electroluminescent layer, is determined by the amount of radiation on the photoconductor. Then the electrodes adjacent the electroluminescent layer are shorted by the commutation means. The dissipation of the charge on the capacitor produces a change in electric field across the electroluminescent phosphor causing it to emit light in accordance with the rate of field variation. With this sort of device the photoconductive layer may be made quite thin 'because there is no critical relationship between the impedances of the two strata. As a result, such a device is capable of high sensitivity. However, the problem with a device such as this is that a rapid and accurate means of commutating must be provided for the purpose described.

Referring now to FIGS. 5 and 6 there is shown a display screen having an electroluminescent cell 11 as was described in `connection with FIG. 1 and having thereon a support member 50` of similar saw tooth configuration as Ithat described in connection with FIG. l but having only one saw tooth projection for a row of light elements.

On one of the sloping surfaces of the V-shaped notch formed by two projections 52 and 53 of the support member 50 there is disposed a layer of photoconductive 'material 55 which may be of the so-rt well known -in the art such as cadmium sulfide and for each light producing element a semiconductive control element 56 like those, 20 and 271, previously described in connection 4with FIG. 1 is provided in series relationship with the photoconductive material 55 land in parallel relationship with the light producing cell 11. It is usually desirable in this embodiment that the control elements 56 be shielded from radiation by some encapsulating material 65 such as a polyester resin.

Conductive members 57 connect together one side of each of the photoconductive members 55 and connect them to one side of a D.C. power source 58 by lead 59. The opposite side of the D.C. source S8 is connected by lead 60 to the front electrode 12 of the electroluminescent cell 1=1 and also, lby lead 61, to conductive members 62 which interconnect the control elements 56. Another set of conductors 63 joins each control element 56 with la photoconductive member 55 and, by conductors 64 to the back electrodes 16. FIG. 7 shows the circuit schematic of a single element of this structure.

In the operation of this device the control element S6 provides a commutation action somewhat similar to that occurring in devices in accordance with the teachings of the above-mentioned Patent 2,916,630. During a first 5 period of operation when the control element is in the high resistance condition, as shown in region 41 of the curve in FIG. 3, charge will build up on the electrode 16 between the electroluminescent and photoconductive layers 14 and 55, respectively. The charge will build up until the potential across the control element exceeds its breakover potential at which time the control element will switch to the hyperconductive state as shown in region t3 of the curve in FIG. 3. The charge on the electrode lle is then dissipated and during the period of charge dissipation a time varying electric field is imposed across the electroluminescent layer `14 resulting in light output. After the dissipation of this charge, the current through the control element 56 is reduced to such a low level that the element reverts to the high resistance state again and the operating cycle is repeated.

'It will be noted that in the device of FIG. 7, the electroluminescent capacitor 1l is charged to the breakover voltage of the control element 56 in each period of operation while the rate of charging is determined by the input radiation on the photoconductor. Therefore, the maximum voltage is fixed but the period of charging varies. In devices as described in Patent 2,916,630, on the other hand, the period of operation is fixed by the mechanical commutator and the maximum voltage on the electroluminescent capacitor varies in accordance with input radiation.

The devices of FIGS. l and 5 are susceptible to fabrication in other forms. For example, the control elements may be fabricated in a continuous strip which extends over a whole row of light producing elements. The fabrication of such a strip of elements may be carried out in accordance with the techniques described in copending application Serial No. 860,174, filed December 17, 1959, by R. L. Longini.

While the present invention has been shown and described in only certain forms, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various changes and modifications without departing from the spirit and scope thereof.

I claim. as my invention:

l. A display device comprising a body of phosphor material having the property of exhibiting electrolurninescence when subjected to a varying electric field and means to vary the electric field across said body in response to an applied radiation signal, said means including at least one semi-conductive control element electrically coupled to said body of phosphor material and responsive to an applied energy signal to switch from a first stable state of low conductivity to a second stable state of hyperconductivity.

2. A display device comprising a body of phosphor material having the property of exhibiting electroluminescence when subjected to a varying electric field and means to vary the electric field across said body in response to an applied radiation signal, said means including one or more semi-conductor control elements electrically coupled to said body of phosphor material and responsive to an applied energy signal to switch from a rst stable state of high resistance to a second stable state of hyperconductivity with a transition region of negative resistance therebetween.

3. A display device comprising a body of phosphor material having the property of exhibiting electroluminescence when subjected to a varying electric field and means to vary the electric field across said body in response to an applied radiation signal, said means including a pair of light sensitive semiconductive control elements electrically coupled to said phosphor layer and responsive to applied radiation to switch from a first stable state of low conductivity to a second stable state of high conductivity.

4. A display device comprising a body of phosphor material having the property of exhibiting electroluminescence when subjected to a varying electric field and means to vary the electric field across said body in response to an applied radiation signal, said means including a pair of light sensitive semiconductive control elements electrically coupled to said phosphor layer and responsive to applied radiation to switch from a first stable state of low conductivity to a second stable state of high conductivity, a potential source coupled across said body of phosphor material and said semiconductive control elements, said source supplying a time varying potential having a period of the order of the R-C time constant of said body of phosphor material and said semiconductive control elements.

5. A display device comprising a layer of phosphor material having the property of exhibiting electroluminescence when subjected to a varying electric field, a light transmissive conductive layer disposed on one surface of said electroluminescent layer, a second conductive layer disposed on the opposite surface of said electroluminescent layer, a support member disposed on the surface of said electroluminescent layer having said second conductive layer thereon, means operable to vary the electric field across said electroluminescent layer in response to an applied radiation signal, said means including one or more semiconductor control elements disposed on said support member and electrically coupled to said second conductive layer, said semiconductor control elements being responsive to an applied energy signal to switch from a first stable state of high resistance to a second stable state of hyperconductivity with a transition region of negative resistance therebetween.

6. A display device comprising a first supportingy member of light transmissive material, a light transmissive conductive layer disposed on said first supporting member, a layer of phosphor material having the property of exhibiting electroluminescence when subjected to a varying electric field disposed on said light transmissive conductive layer, a plurality of elementary electrodes disposed on said electroluminescent layer, a second support member of opaque insulating material disposed on said plurality of electrodes, a plurality of semiconductor control elements one or more of which are electrically coupled to each of said plurality of electrodes, said semiconductor control elements responsive to an applied energy signal to switch from a first stable state of high resistance to a second stable state of hyperconductivity with a transition region of negative resistance therebetween.

7. A display device comprising a layer of phosphor material having the property of exhibiting electroluminescence when subjected to a varying electric field, a light transmissive conducting layer disposed on one surface of said electroluminescent layer, a second conductive layer disposed on the opposite surface of said electroluminescent layer, a support member of opaque material disposed on said opposite surface of said electroluminescent layer and having thereon a pair of light sensitive semiconductive control elements electrically coupled to said second conductive layer and responsive to applied radiation to switch from a first stable state of low conductivity to a second stable state of high conductivity.

8. A display device comprising a layer of phosphor material having the property of exhibiting electroluminescence when subjected to a varying electric field, a light transmissive conducting layer disposed on one surface of said electroluminescent layer, a second conductive layer disposed on the opposite surface of said electroluminescent layer, a support mem-ber of opaque material disposed 6 on said opposite surface of said electroluminescent layer and having thereon a pair of light sensitive semiconductive control elements electrically coupled to said second conductive layer and responsive to applied radiation to switch from :a first stable state of low conductivity to a second stable state of high conductivity, a source of time varying potential having one terminal coupled to said light transmissive conductive layer and the opposite terminal coupled to said semiconductor control elements.

9. A display device comprising a body of phosphor material having the property of exhibiting electroluminescence when subjected to a varying electric field and means to vary the electric field across said body in response to an applied radiation signal, said means including a body of photoconductive material having the property of exhibiting a reduction in impedance when exposed to radiation coupled in series relationship with said electroluminescent body and a semiconductive control element coupled in parallel relationship with said electroluminescent body and responsive to an applied electrical signal to switch from a first stable state of low conductivity to a second stable state of high conductivity.

10. A display device comprising a layer of phosphor material having the property of exhibiting electroluminescence when subjected to a varying electric field, a light transmissive conducting layer disposed on one surface of said electroluminescent layer, a second conductive layer disposed on the opposite surface of said electroluminescent layer, a support member of insulating material disposed on said second conductive layer, means to vary the electric field across said body in response to an applied radiation signal, said means including a body photoconductive material having the property of reducing in impedance when exposed to radiation and coupled by conductive means extending through said support member to said second conductive layer, a semiconductive control element electrically coupled to said Aphotoconductive body and to said back conductive electrode and responsive to and applied electrical signal to switch from a first stable state of low conductivity to a second stable state of high conductivity.

11. A display device comprising a layer of phosphor material having the property of exhibiting electroluminescence when subjected to a varying electric field, a light transmissive conducting layer disposed on one surface of said electroluminescent layer, a second conductive layer disposed on the opposite surface of said electroluminescent layer, a support member of insulating material disposed on said second conductive layer, means to vary the electric eld across said body in response to an applied radiation signal, said means including a body of Vphotoconductive material having the property of reducing in impedance when exposed to radiation and coupled by conductive means extending through said support member to said second conductive layer, a semiconductive control element electrically coupled to said photoconductive body and to said back conductive electrode and responsive to an applied electrical signal to switch from afirst stable state of low conductivity to a second stable state of high conductivity, a source of direct current potential coupled across the photoconductive body and said electroluminescent layer and semiconductor switching element in parallel combination.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A DISPLAY DEVICE COMPRISING A BODY OF PHOSPHOR MATERIAL HAVING THE PROPERTY OF EXHIBITING ELECTROLUMINESCENCE WHEN SUBJECTING TO A VARYING ELECTRIC FIELD AND MEANS TO VARY THE ELECTRIC FIELD ACROSS SAID BODY IN RESPONSE TO AN APPLIED RADIATION SINGNAL, SAID MEANS INCLUDING AT LEAST ONE SEMI-CONDUCTIVE CONTROL ELEMENT ELECTRICALLY COUPLED TO SAID BODY PF PHOSPHOR MATERIAL AND RESPONSIVE TO AN APPLIED ENERGY SIGNAL TO SWITCH FROM A FIRST STABLE STATE OF LOW CONDUCTIVITY TO A SECOND STABLE STATE OF HYPERCONDUCTIVITY. 